Noise Reduction System

ABSTRACT

A noise reduction system is used in a BTSC system to reduce noise of an audio signal. The noise reduction system has an audio spectral compressing unit that has a filter and a memory in the approach of the digital processing. The filter is arranged to filter an input signal according to a transfer function, a variable d, and several parameters b 0 /a 0 , a 0 /b 0 , b 1 /b 0  and a 1 /a 0 . The memory is arranged to store the parameters.

BACKGROUND

1. Field of Invention

The present invention relates to a noise reduction system, and moreparticularly relates to a noise reduction system used in the BTSCsystem.

2. Description of Related Art

In the 1980's, the United States FCC (Federal Communications Commission)adopted new regulations covering the audio portion of television signalsthat permitted television programs to be broadcast and received withbi-channel audio. In those regulations, the FCC recognized and gavespecial protection to a method of broadcasting additional audio channelsthat is also called the BTSC (Broadcast Television System Committee)system. The BTSC system defines MTS (multi-channel television sound)transmission and its audio processing requirements.

FIG. 1 shows a noise reduction system of the prior art. The noisereduction system 110 is used in the BTSC system to reduce noise of anaudio signal and generate an encoded audio signal during an encodingprocess in the approach of the digital processing. The noise reductionsystem 110 has an audio spectral compressing unit 120, a widebandcompression circuit 150, and a multiplier 160 when the noise reductionsystem 110 is used in the encoding process. The audio spectralcompressing unit 120 has a filter 130 and a memory 140. The filter 130filters an input signal according to a transfer function, a variable d,and several parameters (coefficients of the transfer function). Thetransfer function is:

$\begin{matrix}{{H(z)} = \frac{\frac{b_{0}}{a_{0}} + {\frac{b_{1}}{a_{0}}z^{- 1}}}{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}} & (1)\end{matrix}$

The memory 140 is arranged to store the parameters. When the variable dis greater than zero, i.e. d>0, the memory 140 outputs the parameters(b₀/a₀)⁺, (b₁/b₀)⁺ and (a₁/a₀)⁺ to the filter 130; when the variable dis less than zero, i.e. d<0, the memory 140 outputs the parameters(b₀/a₀)⁻, (b₁/b₀)⁻ and (a₁/a₀)⁻ to the filter 130.

From this transfer function (1), the memory 140 needs to store 6parameters (b₀/a₀)⁺, (b₁/b₀)⁺, (a₁/a₀)⁺, (b₀/a₀)⁻, (b₁/b₀)⁻ and(a₁/a₀)⁻. Because the cost of the memory is proportional to the capacityof the memory, a noise reduction system with a memory of smallercapacity is needed.

SUMMARY

It is therefore an aspect of the present invention to provide a noisereduction system with a memory of smaller capacity.

It is therefore another aspect of the present invention to provide anaudio processing unit with a memory of smaller capacity.

According to one preferred embodiment of the present invention, thenoise reduction system is used in the BTSC system to reduce the noise ofan audio signal during an encoding process in the approach of thedigital processing. The noise reduction system has an audio spectralcompressing unit when the noise reduction system is used in the encodingprocess. The audio spectral compressing unit has a filter and a memory.The filter is arranged to filter an input signal according to a transferfunction, a variable d, and several parameters b₀/a₀, a₀/b₀, b₁/b₀ anda₁/a₀, wherein the transfer function is:

when the variable d is greater than zero:

${H(z)} = {\frac{b_{0}}{a_{0}} \times \frac{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}}$

when the variable d is less than zero:

${H(z)} = {\frac{a_{0}}{b_{0}} \times \frac{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}}$

The memory is arranged to store the parameters. When the variable d isgreater than 0, i.e. d>0, the memory outputs the parameters b₀/a₀, b₁/b₀and a₁/a₀ to the filter; when the variable d is less than 0, i.e. d<0,the memory outputs the parameters a₀/b₀, b₁/b₀ and a₁/a₀ to the filter.

According to another preferred embodiment of the present invention, theaudio processing unit is used in the BTSC system to process an audiosignal of an encoding process in the approach of the digital processing.The audio processing unit has a multiplexer, a memory and a filter. Themultiplexer is arranged to select and output several parameter addressesaccording to a variable d. The memory is arranged to receive theparameter addresses and output several parameters b₀/a₀, a₀/b₀, b₁/b₀and a₁/a₀. When the variable d is greater than 0, i.e. d>0, the memoryoutputs the parameters b₀/a₀, b₁/b₀ and a₁/a₀; when the variable d isless than 0, i.e. d<0, the memory outputs the parameters a₀/b₀, b₁/b₀and a₁/a₀. When the audio processing unit is used in the encodingprocess, the filter is arranged to filter an input signal according to atransfer function, the variable d, and the parameters b₀/a₀, a₀/b₀,b₁/b₀ and a₁/a₀, wherein the transfer function is:

when the variable d is greater than 0, i.e. d>0:

${H(z)} = {\frac{b_{0}}{a_{0}} \times \frac{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}}$

when the variable d is less than 0, i.e. d<0:

${H(z)} = {\frac{a_{0}}{b_{0}} \times \frac{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}}$

It is to be understood that both the foregoing general description andthe following detailed description are examples and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 shows a noise reduction system of the prior art.

FIG. 2A shows a noise reduction system of a first preferred embodimentof the present invention.

FIG. 2B shows a noise reduction system of a second preferred embodimentof the present invention.

FIG. 3A shows a digital audio processing unit of a third preferredembodiment of the present invention.

FIG. 3B shows a digital audio processing unit of a fourth preferredembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

This invention offers a noise reduction system and an audio processingunit used in the BTSC system to reduce noise during an encoding processor a decoding process in the approach of the digital processing. Thefilter of the noise reduction system and the audio processing unit usesa new transfer function with fewer parameters (coefficients of thetransfer function) to reduce required memory capacity. Using thisdevice, parameters are more economically stored in the memory.

When the filter of the noise reduction system is used for an encodingprocess, the transfer function is:

S(f,b)=[1+(f/20.1[kHz])(b+51)/(b+1)]/[1+(f/20.1[kHz])(1+51b)/(b+1)]  (2a)

When the filter of the noise reduction system is used for a decodingprocess, the transfer function is:

S ⁻¹(f,b)=[1+(f/20.1[kHz])(1+51b)/(b+1)]/[1+(f/20.1[kHz])(b+51)/(b+1)]  (2b)

Wherein ‘f ’ is the frequency of processing signal, ‘b’ is thetime-weighted root mean square of the encoded audio signal.

In order to apply the transfer functions (2 a) and (2 b) in a digitalaudio processor, the S(f,b) and S⁻¹(f,b) have to be bilinear transformedinto Z domain. Therefore set b=10^((d/20)), i.e. d=20 log (b), and thetransfer functions (2 a) and (2 b) respectively become:

S(Z,b)=[2πf(Z+1)(b+1)+2f _(s)(Z−1)(b+51)]/[2πf(Z+1)(b+1)+2f_(s)(Z−1)(1+51 b)]  (3a)

S ⁻¹(Z,b)=[2πf(Z+1)(1+b)+2f _(s)(Z−1)(1+51b)]/[2πf(Z+1)(1+b)+2f_(s)(Z−1)(b+51)]  (3b)

where f=20.1 kHz, f_(s) is the sampling frequency.

In the transfer function (3 b), S⁻¹(Z,b) is equal to S (Z,b⁻¹). Thus,the transfer functions (3 a) and (3 b) are set to be:

When d>0,S(Z,b)=H(Z)=(b ₀ +b ₁ Z ⁻¹)/(a ₀ +a ₁ Z ⁻¹)  (4a)

When d<0,S(Z,b ⁻¹)=H ⁻¹(Z)=(a ₀ +a ₁ Z ⁻¹)/(b ₀ +b ₁ Z ⁻¹)  (4b)

In the transfer function (1) of the prior art, the memory needs to store6 parameters. In order to reduce the amount of the parameters, thetransfer functions (4 a) and (4 b) are transformed to be:

when the variable d is greater than 0, i.e. d>0:

$\begin{matrix}{{H(z)} = {\frac{b_{0}}{a_{0}} \times \frac{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}}} & \left( {5a} \right)\end{matrix}$

when the variable d is less than 0, i.e. d<0:

$\begin{matrix}{{H(z)} = {\frac{a_{0}}{b_{0}} \times \frac{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}}} & \left( {5a} \right)\end{matrix}$

FIG. 2A shows a noise reduction system of a first preferred embodimentof the present invention. The noise reduction system 210 is used in theBTSC system to reduce noise of an audio signal during an encodingprocess and to generate an encoded audio signal. The noise reductionsystem 210 has an audio spectral compressing unit 220 a when the noisereduction system 210 is used in the encoding process. The audio spectralcompressing unit 220 a has a filter 230 and a memory 240. The filter 230of the audio spectral compression unit 220 a is arranged to filter aninput signal and generate the encoded audio signal according to atransfer function, a variable d, and several parameters (coefficients ofthe transfer function) b₀/a₀, a₀/b₀, b₁/b₀ and a₁/a₀. The transferfunctions are (5a) and (5 b):

when the variable d is greater than 0, i.e. d>0:

$\begin{matrix}{{H(z)} = {\frac{b_{0}}{a_{0}} \times \frac{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}}} & \left( {5a} \right)\end{matrix}$

when the variable d is less than 0, i.e. d<0:

$\begin{matrix}{{H(z)} = {\frac{a_{0}}{b_{0}} \times \frac{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}}} & \left( {5a} \right)\end{matrix}$

The memory 240 is arranged to store the parameters, when the variable dis greater than 0, i.e. d>0, the memory outputs the parameters b₀/a₀,b₁/b₀ and a₁/a₀ to the filter 230; when the variable d is less than 0,i.e. d<0, the memory outputs the parameters a₀/b₀, b₁/b₀ and a₁/a₀ tothe filter 230.

From this transfer function, the memory just needs to store 4 parametersb₀/a₀, a₀/b₀, b₁/b₀ and a₁/a₀. Furthermore, the parameter a₀/b₀ can begenerated from the parameter b₀/a₀ by using hardware (such as acircuit). Therefore, compared with the conventional memory that stores 6parameters in the noise reduction system, this memory just needs tostore 3˜4 parameters. This memory needs only ½˜⅔ capacity ofconventional memory.

The variable d is an address of the memory, and the variable d is equalto 20 μg (the time-weighted root mean square of the encoded audiosignal). In order to get equal filter frequency response of theparameter d>0 and d<0, the range of the variable d is about ±35[decibelERMS] to about ±45[decibel ERMS].

When the noise reduction system 210 is used in the encoding process, awideband compression unit 250 a coupled to the memory 240 and the filter230 in the noise reduction system compresses the encoded audio signalinto a wideband compression signal. The noise reduction system 210further has a multiplier 260 coupled to the wideband compression unit250 a and the filter 230. The multiplier 260 generates the input signalby multiplying the audio signal with the wideband compression signal.

In real products, the memory 240 in the noise reduction system 210 isconventionally implemented with a ROM table, such as a look up ROMtable.

FIG. 2B shows a noise reduction system of a second preferred embodimentof the present invention. The noise reduction system 210 is used in theBTSC system to reduce noise of an encoded audio signal during a decodingprocess and to generate an audio signal in the approach of the digitalprocessing. The difference between FIG. 2A and FIG. 2B is that the noisereduction system 210 of FIG. 2B has an audio spectral expansion unit 220b when the noise reduction system is used in the decoding process.

The filter 230 of the audio spectral expansion unit 220 b is arranged tofilter the encoded signal and generate an output signal according to aninverse of the transfer function, the variable d, and the parametersb₀/a₀, a₀/b₀, b₁/b₀ and a₁/a₀ as described in FIG. 2A.

When the noise reduction system 210 is used in the decoding process, awideband expansion unit 250 b coupled to the memory 240 in the noisereduction system expands the encoded audio signal to be a widebandexpansion signal. The noise reduction system 210 further has amultiplier 260. The multiplier 260 coupled to the wideband expansionunit 250 b and the filter 230 is arranged to multiply the output signalwith the wideband expansion signal to be the audio signal.

FIG. 3A shows a digital audio processing unit of a third preferredembodiment of the present invention. The digital audio processing unit310 a is used in the BTSC system to process an audio signal of anencoding process and to generate an encoded audio signal. The digitalaudio processing unit 310 a has a multiplexer 320, a memory 340 and afilter 330. The multiplexer 320 is arranged to select and output severalparameter addresses according to a variable d. When the variable d isgreater than 0, i.e. d>0, the multiplexer 320 outputs addresses ofparameters b₀/a₀, b₁/b₀ and a₁/a₀. When the variable d is less than 0,i.e. d<0, the multiplexer 320 outputs addresses of parameter a₀/b₀,b₁/b₀ and a₁/a₀.

The memory 340 coupled to the multiplexer is arranged to receive theparameter addresses and output several parameters b₀/a₀, a₀/b₀, b₁/b₀and a₁/a₀. When the variable d is greater than 0, i.e. d>0, the memoryoutputs the parameters b₀/a₀, b₁/b₀ and a₁/a₀ to the filter 330. Whenthe variable d is less than 0, i.e. d<0, the memory outputs theparameters a₀/b₀, b₁/b₀ and a₁/a₀ to the filter 330.

The filter 330 is coupled to the memory. When the audio processing unit330 is used in the encoding process, the filter 330 is arranged tofilter an input signal according to a transfer function, the variable d,and the parameters b₀/a₀, a₀/b₀, b₁/b₀ and a₁/a₀. The transfer functionsare equations (5a) and (5 b) as described above.

In the digital audio processing unit 310 a, the multiplexer 320 can beconfigured in the memory, and the variable d is an address of thememory. Furthermore, the variable d is 20 log (the time-weighted rootmean square of the encoded audio signal). In order to get equal filterfrequency response for the parameter d>0 and the parameter d<0, therange of the variable d is about 35[decibel ERMS] to about ±45[decibelERMS].

The digital audio processing unit 310 a further has a gain device 370, aspectral bandpass filter 380, and an energy level detecting device 390.The gain device 370 is coupled to the filter 330 to receive and increasethe gain of the encoded audio signal. The spectral bandpass filter 380is coupled to the gain device 370 to generate a spectral signalaccording to the encoded audio signal with increasing gain. The energylevel detecting device 390 is coupled to the spectral bandpass filter380 and the multiplexer 320 to generate the variable d according to thespectral signal.

When the digital audio processing unit 310 a is used in the encodingprocess, a wideband compression unit 350 a coupled to the gain deviceand the filter 330 compresses the encoded audio signal into a widebandcompression signal. The digital audio processing unit 310 a further hasa multiplier 360 coupled to the wideband compression unit 350 a and thefilter 330. The multiplier 360 generates the input signal by multiplyingthe audio signal with the wideband compression signal.

In real products, the memory 340 in the digital audio processing unit310 a is conventionally implemented by a ROM table, such as a look upROM table.

FIG. 3B shows a digital audio processing unit of a fourth preferredembodiment of the present invention. The digital audio processing unit310 b is used in the BTSC system to process an encoded audio signal of adecoding process and to generate an audio signal.

When the digital audio processing unit 310 b is used in the decodingprocess, the filter 330 is arranged to filter the encoded signal andgenerate an output signal according to an inverse of the transferfunction, the variable d, and the parameters b0/a0, a0/b0, b1/b0 anda1/a0.

When the digital audio processing unit 310 b is used in the decodingprocess, the digital audio processing unit 310 b further has a widebandexpansion unit 350 b coupled to the gain device 370 to expand theencoded audio signal into a wideband expansion signal. The digital audioprocessing unit 310 b further has a multiplier 360. The multiplier 360coupled to the wideband expansion unit 350 b and the filter 330generates the audio signal by multiplying the output signal with thewideband expansion signal.

Using the noise reduction system or the audio processing unit describedabove, this memory needs only ½˜⅔ capacity of a conventional memory. Theaudio processing data of the multimedia in real life is very huge, thenoise reduction system and the audio processing unit can reduce thenecessary memory capacity.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A noise reduction system used in a BTSC system to reduce noise of anaudio signal during an encoding process in the approach of the digitalprocessing, characterized by an audio spectral compressing unit,wherein: the audio spectral compressing unit comprises: a filterarranged to filter an input signal according to a transfer function, avariable d, and a plurality of parameters b₀/a₀, a₀/b₀, b₁/b₀ and a₁/a₀,wherein the transfer function is: when the variable d is greater than 0:${H(z)} = {\frac{b_{0}}{a_{0}} \times \frac{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}}$when the variable d is less than 0:${H(z)} = {\frac{a_{0}}{b_{0}} \times \frac{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}}$; and a memory arranged to store the parameters, when the variable d isgreater than 0, the memory outputs the parameters b₀/a₀, b₁/b₀ and a₁/a₀to the filter; when the variable d is less than 0, the memory outputsthe parameters a₀/b₀, b₁/b₀ and a₁/a₀ to the filter.
 2. The noisereduction system claimed in claim 1, wherein the variable d is anaddress of the memory.
 3. The noise reduction system claimed in claim 1,wherein the variable d is equal to 20 log (the time-weighted root meansquare of the encoded audio signal).
 4. The noise reduction systemclaimed in claim 3, wherein the range of the variable d is about±35[decibel ERMS] to about ±45[decibel ERMS].
 5. The noise reductionsystem claimed in claim 1, wherein when the noise reduction system isused in the encoding process, further comprising a wideband compressionunit in the noise reduction system to compress the encoded audio signalto be a wideband compression signal.
 6. The noise reduction systemclaimed in claim 5, further comprising a multiplier in the noisereduction system to multiply the audio signal with the widebandcompression signal to be the input signal.
 7. The noise reduction systemclaimed in claim 1, wherein the memory is a ROM table.
 8. The noisereduction system claimed in claim 1, wherein the noise reduction systemis used in the BTSC system to reduce noise of an encoded audio signalduring a decoding process in the approach of the digital processing. 9.The noise reduction system claimed in claim 8, wherein the noisereduction system comprises an audio spectral expansion unit when thenoise reduction system is used in the decoding process, the filter ofthe audio spectral expansion unit is arranged to filter the encodedsignal according to an inverse of the transfer function, the variable d,and the parameters b0/a0, a0/b0, b1/b0 and a1/a0.
 10. The noisereduction system claimed in claim 8, wherein when the noise reductionsystem is used in the decoding process, further comprising a widebandexpansion unit in the noise reduction system to expand the encoded audiosignal to be a wideband expansion signal.
 11. The noise reduction systemclaimed in claim 8, wherein the multiplier is arranged to multiply theoutput signal with the wideband expansion signal to be the audio signal.12. A digital audio processing unit used in a BTSC system to process anaudio signal during an encoding process, the audio processing unitcomprising: a multiplexerarranged to select and output a plurality ofparameter addresses according to a variable d; a memory coupled to themultiplexer and arranged to receive the parameter addresses and output aplurality of parameters b₀/a₀, a₀/b₀, b₁/b₀ and a₁/a₀, when the variabled is greater than 0, the memory outputs the parameters b₀/a₀, b₁/b₀ anda₁/a₀; when the variable d is less than 0, the memory outputs theparameters a₀/b₀, b₁/b₀ and a₁/a₀; and a filter coupled to the memory,wherein when the audio processing unit is used in the encoding process,the filter is arranged to filter an input signal according to a transferfunction, the variable d, and the parameters b₀/a₀, a₀/b₀, b₁/b₀ anda₁/a₀, wherein the transfer function is: when the variable d is greaterthan 0:${H(z)} = {\frac{b_{0}}{a_{0}} \times \frac{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}}$when the variable d is less than 0:${H(z)} = {\frac{a_{0}}{b_{0}} \times \frac{1 + {\frac{a_{1}}{a_{0}}z^{- 1}}}{1 + {\frac{b_{1}}{b_{0}}z^{- 1}}}}$13. The digital audio processing unit claimed in claim 12, wherein themultiplexer is configured in the memory, and the variable d is anaddress of the memory.
 14. The digital audio processing unit claimed inclaim 12, wherein the variable d is 20 log (the time-weighted root meansquare of the encoded audio signal).
 15. The digital audio processingunit claimed in claim 14, wherein the range of the variable d is about±35[decibel ERMS] to about ±45[decibel ERMS].
 16. The digital audioprocessing unit claimed in claim 12, further comprising a gain devicearranged to receive the encoded audio signal and increase the gain ofthe encoded audio signal.
 17. The digital audio processing unit claimedin claim 16, further comprising a spectral bandpass filter coupled tothe gain device to generate a spectral signal according to the encodedaudio signal with increasing gain.
 18. The digital audio processing unitclaimed in claim 17, further comprising an energy level detecting devicecoupled to the spectral bandpass filter and the multiplexer to generatethe variable d according to the spectral signal.
 19. The digital audioprocessing unit claimed in claim 12, wherein when the audio processingunit is used in the encoding process, further comprising a widebandcompression unit coupled to the gain device and the filter to compressthe encoded audio signal into a wideband compression signal.
 20. Thedigital audio processing unit claimed in claim 19, further comprising amultiplier coupled to the wideband compression unit and the filter tomultiply the audio signal with the wideband compression signal to be theinput signal.
 21. The digital audio processing unit claimed in claim 10,wherein the memory is a ROM table.
 22. The digital audio processing unitclaimed in claim 12, wherein the audio processing unit is used in theBTSC system to process an encoded audio signal of a decoding process.23. The digital audio processing unit claimed in claim 22, wherein whenthe audio processing unit is used in the decoding process, the filter isarranged to filter the encoded signal and generate an output signalaccording to an inverse of the transfer function, the variable d, andthe parameters b0/a0, a0/b0, b1/b0 and a1/a0.
 24. The digital audioprocessing unit claimed in claim 22, wherein when the audio processingunit is used in the decoding process, further comprising a widebandexpansion unit coupled to the gain device to expand the encoded audiosignal into a wideband expansion signal.
 25. The digital audioprocessing unit claimed in claim 22, wherein the multiplier coupled tothe wideband expansion unit is arranged to multiply the output signalwith the wideband expansion signal to be the audio signal.